This invention pertains to the art of power supplies and more particularly to regulated constant output voltage sinusoidal input current power supplies.
The invention maintains output voltage constant and input current sinusoidal during both slow varying changes in the output load and large fast transient load steps and load dumps, through automatic responses within the power factor correction section of the regulated power supply, and will be described with particular reference thereto. It will be appreciated, however, that the invention has broader applications that may be advantageously employed in other environments and applications that impose similar constraints.
Power supplies including regulated power supplies are in widespread use in a variety of environments including supplying power in the telecommunication industry. In this, as well as other industries where significant load changes are common and maintaining low input current distortion is important; regulated power supplies which eliminate distortion are desirable. Therefore, such regulated power supplies include power factor correction which virtually eliminates input current distortion allowing steady undistorted input currents to be generated. Another requirement for such regulated power supplies is that they respond to fast transient changes in the load so that the regulated power supply output is maintained within a predetermined acceptable operating range. Unfortunately, obtaining low signal distortion and fast response to load changes are two diametrically opposed goals when implemented in regulated power supplies. The characteristics to ensure low signal distortion act against providing proper response to fast large load changes. For a regulated power supply to generate a low distortion signal, a very low bandwidth amplifier is needed in the power factor correction section of the power supply. Such a low bandwidth amplifier filters out AC component ripple existing on an output bank capacitor of the regulated power supply. This filtered signal works to eliminate unwanted distortions. However, by using such a low bandwidth amplifier, when a large fast load change occurs at the output of the regulated power supply it is not possible for the power factor correction section to respond in a manner to keep the output voltage within predetermined limits.
While power factor correction configured to provide low distortion will react to load changes, it will react very slowly, so that by the time a new load level is reached, either high or low voltage protection alarms will be reached, and the power supply will shut down.
On the otherhand, if, instead of low signal distortion, a circuit for proper transient response were desired, a large bandwidth or fast reacting power factor correction would be desirable. In such a situation, the regulated power supply will react quickly to load changes quickly bringing the output voltage within desirable parameters. In this situation, when fast load changes occur, the likelihood of the output voltage reaching alarm limits can be avoided.
Previous systems have addressed these issues by attempting to provide compromises between the opposed goals. One such system exemplified in U.S. Pat. No. 5,006,975 to Neufeld uses a sample and hold configuration. However, a drawback of such a system is that after the sample is taken the power supply is forced to use that sample for at least 1/2 cycle, or for a 60 Hertz system for approximately 8 milliseconds. Therefore, depending upon the ability of the power supply to maintain its output voltage for 8 ms, which is based upon the amount of energy stored in its output capacitance, the output voltage could be out of the desired range within this time period.
Other systems attempting to address these conflicting goals make compromises in the selection of components for the output voltage control circuit of the regulated power supply. However, by implementing such compromises, optimum performance in distortion, output voltage regulation and transient response cannot be achieved simultaneously.
Power factor correction is desirable in an off-line regulated power supply since it works to minimize wasted energy. If a distorted wave shape is present in the distribution wiring, there will be less usable power at the output for a given current. Another problem which exists with poor power factor is especially relevant to three-phase systems. In a balanced undistorted three-phase system the neutral wire will normally carry a minimum current. In an unbalanced or distorted system such as one with poor power factor, the neutral becomes a high-current carrier, increasing the chances of damage such as by fire.
Further, when power factor correction is provided in a power supply, the smooth sinewave input as shown in FIG. 1A can be obtained. However, a circuit without power factor correction can generate a highly distorted wave form which generates bursts of currents as illustrated in FIG. 1B. Such a wave form generates large amounts of heat that is wasted energy. Further, if there is a weak generating system, such as an inverter or engine generator, the input current will flattop the power line voltage as shown in FIG. 1C.
It has, therefore, been deemed desirable to design a power supply with power factor correction that eliminates the compromises and tradeoffs of prior systems and which achieves the goals of low input current distortion and good transient response. Further, the power correction circuitry should be easily implemented into known power supply configurations in an economical manner. Still further, the power factor correction will automatically adjust its operation dependent upon whether slowly varying load changes or large fast transient load changes occur at the output.